The main components of bypass dusts are CaO, Al2O3, SiO2 and Fe2O3 as well as condensed impurities such as Na2O, K2O, SO3 and Cl.
In cement production, the hot exhaust gases from the kiln are used to preheat the raw meal. The vaporous components such as Na2O, K2O, SO3 and Cl, which are contained in the exhaust gas, condense due to cooling and are returned into the kiln along with the raw meal. An internal circulation of such substances which evaporate in the kiln and condense out of the exhaust gas during preheating of the raw meal is thus created in cement production. Along with the raw meal or the fuels, new, unavoidable components are constantly introduced such that the described circulations will become increasingly enriched unless a portion of the hot exhaust gases from the kiln is constantly withdrawn, and hence also the evaporated impurities are proportionally eliminated from the production process. This partial gas stream is cooled and dedusted, the dust being referred to as cement kiln gas bypass dust or briefly bypass dust in the following, wherein the unavoidable components condense on the dust grains and are separated from the exhaust gas along with the dust. The occurring bypass dust can be used as a cement grinding additive depending on its composition and the desired cement quality.
The use of alternative fuels or the use of specific raw materials in the cement production process, and the associated introduction of additional unavoidable components, involve the formation of increasing amounts of bypass dusts, which cannot be completely utilized without elaborate processing and are therefor partially dumped by many cement producers, thus constituting a considerable environmental impact. Dumping is also disadvantageous in that it entails accordingly high losses of components suitable for cement production. As a rule, an economically reasonable use of such products in the cement industry fails because of alkali, chloride, sulfur or heavy metal contents that are too high.